Calculating the length of a year

[Steve Schlaifer x43171 301/167]

In the Explanatory Supplement to the Astronomical Ephemeris and American
Ephemeris and Nautical Almanac, page 412, Section 14C,

	"The Gregorian reform of the Julian calendar consisted of:

	(i) ommitting 10 days from the calendar reckoning, the day next after 
	1582 October 4 being designated 1582 October 15, for the purpose of 
	restoring the date of the actual vernal equinox to March 21;

	(ii) adopting a different rule for leap year, by omitting the 
	intercalary day in centurial years that are not divisible by 400, such
	as 1700, 1800, 1900, and 2100, in order to correct the error of the
	Julian calendar where an intercalary day is inserted every four years;

	(iii) fixing rules for determining the date of Easter in the revised
	calendar.

	The mean length of the Gregorian calendar year is 365.2425 days.  At
	the completion of a 400-year calendar cycle, the cumulative discrepancy
	with the tropical year is only a few hours."

[ Note that there is no mention of any further correction; just a note that
there is a small cumulative error. -rss ]

Later, on page 413, the Explanatory Supplement notes that

	"At a meeting of a Congress of the Orthodox Oriental Churches held in
	Constantinople in May, 1923, the Julian calendar was replaced by a
	modified Gregorian calendar in which century years are leap years only
	when division of the century number by 9 leaves a remainder of either
	2 or 6, and Easter is determined by the astronomical Moon for the
	meridian of Jerusalem; see Milankovitch (Milankovitch, M. Das Ende
	des julianischen Kalenders und der neue Kalender der orientalischen
	Kirchen. _Ast. Nach._, *220*, 379-384, 1924).

Note that this was long after Europe, the British colonies (including what
became the U.S. and Canada), and, I presume, the Spanish and Portugese colonies
had already adopted the unmodified Gregorian calendar and was only adopted by
the Orthodox Oriental Churches (Greek Orthodox) as near as I can tell.
-- 

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